Generally, contact lenses in wide use fall into three categories: (1) hard lenses formed from materials prepared by polymerization of acrylic esters, such as polymethyl methacrylate (PMMA), (2) rigid gas permeable (RGP) lenses formed from silicone acrylates and fluorosilicone methacrylates, and (3) gel, hydrogel or soft type lenses made of polymerized hydrophilic or hydrophobic monomers, such as 2-hydroxyethyl methacrylate (HEMA). The hard acrylic type contact lenses are characterized by low water vapor diffusion constants, resistance to the effects of light, oxygen and hydrolysis, and absorb only minor amounts of aqueous fluids. Because of the durability of hard contact lenses, coupled with their tendency not to absorb appreciable amounts of water, the selection of suitable disinfecting agents, cleaning agents or other lens care compounds is relatively non-critical.
However, unlike hard lenses, soft-type contact lenses have a tendency to bind and concentrate significantly more fluids, environmental pollutants, water impurities, as well as antimicrobial agents and other active ingredients commonly found in lens-care solutions. In most instances, the low levels of the ingredients in lens-care solutions do not lead to eye tissue irritation when used properly. Nevertheless, especially due to the inherent binding action of protein deposits to soft-lens materials, some disinfecting agents and preservatives tend to build up on lens surfaces and may become concentrated to potentially hazardous levels, such that when released could cause corneal inflammation and other eye tissue irritation.
Certain antibacterial agents were found to be more compatible with contact lenses and exhibit less binding on lens surfaces. In one case, it was found that chlorhexidine, a biguanide, binds to soft lens material seven times less than benzalkonium chloride. The presence of proteinaceous oily tear-film deposits on a lens, however, can double the amount of chlorhexidine absorbed on the lens compared to a clean lens. U.S. Pat. No. 4,354,952 discloses very dilute disinfecting and cleaning solutions containing chlorhexidine or its salt in combination with certain amphoteric and non-ionic surfactants. These solutions were found to reduce the amount of binding of chlorhexidine on hydrophilic soft contact lenses. Notwithstanding the reduction in binding achieved by this invention, the use of chlorhexidine did result in certain tradeoffs. The antimicrobial activity of the chlorhexidine may be diminished when used with certain amphoteric surfactants. Furthermore, it was reported that if not used in proper ratio, the surfactant and disinfectant will precipitate unless a non-ionic type surfactant is also employed.
The use of certain ionic polymers in contact-lens cleaning and preserving solutions is also known. For example, U.S. Pat. No. 5,096,607 and WO 94/13774 disclose the use of certain polyquaterniums as antimicrobial agents, typically in amounts less than 100 parts-per-million (ppm) in actual commercial practice. U.S. Pat. No. 4,443,429 to Smith et al. discloses the use in a contact-lens disinfecting solution of a dimethyldiallyammonium chloride homopolymer known as Merquat.RTM. 100 having a molecular weight of about 10,000 to about 1,000,000. Polyquaternium polymers in general are a well known class of polymers, many variations of which are commercially available. The CTFA International Cosmetic Ingredient Dictionary includes polyquaterniums designated Polyquaternium-1 through Polyquaternium 44, and new polyquaterniums are in continuous development.
British Patent 1,432,345 discloses contact lens disinfecting compositions containing a polymeric biguanide and a mixed phosphate buffer. Compositions as disclosed by this patent, however, have corneal staining values of 17% or more, far above that which is desirable for patient acceptability.
U.S. Pat. No. 4,758,595 to Ogunbiyi et al. disclosed that a contact-lens solution containing a polyaminopropyl biguanide (PAPB), also known as polyhexamethylene biguanide (PHMB), has enhanced efficacy when combined with a borate buffer. These disinfecting and preservative solutions are especially noteworthy for their broad spectrum of bactericidal and fungicidal activity at low concentrations coupled with very low toxicity when used with soft-type contact lenses.
U.S. Pat. No. 5,453,435 to Raheja et al., disclosed a preservative system that comprises a combination of chlorhexidine and polyhexamethylene biguanide. This preservative system, used in commercial products for rigid-gas-permeable lenses, was found to exhibit an improved combination of efficacy and low eye irritation.
Compositions containing PHMB and borate have been commercialized in various products, but at levels of about 1 ppm or less for use with soft contact lenses. It is generally desirable to provide the lowest level of a bactericide possible, while maintaining the desirable level of disinfection efficacy, in order to provide a generous margin for safety and comfort.
Some of the most popular products for disinfecting lenses are multipurpose solutions that can be used to clean, disinfect and wet contact lenses, followed by direct insertion (placement on the eye) without rinsing. Obviously, the ability to use a single solution for contact-lens care is an advantage. Such a solution, however, must be particularly gentle to the eye, since, as indicated above, some of the solution will be on the lens when inserted and will come into contact with the eye.
With conventional contact-lens cleaners or disinfectants, including multi-purpose solutions, lens wearers typically need to digitally or manually rub the contact lenses (typically between a finger and palm or between fingers) during treatment of the contact lenses. The necessity for the daily "rubbing" of contact lenses adds to the time and effort involved in the daily care of contact lenses. Many contact-lens wearers dislike having to perform such a regimen or consider it to be an inconvenience. Some wearers may be negligent in the proper "rubbing" regimen, which may result in contact-lens discomfort and other problems. Sometimes rubbing, if performed too rigorously, which is particularly apt to occur with beginning lens wearers, may damage the lenses. This can be problematic when a replacement lens is not immediately available.
Contact lens solutions that qualify as a "Chemical Disinfecting Solution" do not require rubbing to meet biocidal performance criteria (for destroying representative bacteria and fungi) set by the US Food and Drug Administration (FDA) under the Premarket Notification (510k) Guidance Document For Contact Lens Care Products, May 1, 1997. In contrast, a contact-lens solution, referred to as a "Chemical Disinfecting System," that does not qualify as a Chemical Disinfecting Solution, requires a rubbing regimen to pass biocidal performance criteria. Traditionally, multi-purpose solutions (used for disinfecting and wetting or for disinfecting, cleaning, and wetting) have qualified as a Chemical Disinfecting System, but not as a Chemical Disinfecting Solution.
A Chemical Disinfecting Solution would generally require a more efficacious or stronger disinfectant than a Chemical Disinfecting System. The stronger the biocidal effect of a solution, however, the more likely that it may exhibit toxic effects or adversely effect lens-wearer comfort. For example, many very efficacious bactericides used in other contexts, such as mouthwashes, cosmetics, or shampoos, while being sufficiently safe for use in such products, would be too toxic for ophthalmic use, especially for use with soft lenses because of the above-mentioned tendency of soft lenses to bind chemicals and the sensitivity of eye tissues. Similarly, the concentrations of certain bactericides may need to be within lower limits in solutions for use with soft contact lenses than in other products or in solutions for other types of lenses, especially when such solutions are not rinsed from the contact lens before placing the lens in the eye.
It would be desirable to obtain a contact-lens solution that would simultaneously provide both (1) an increased level and/or broader spectrum of biocidal activity, and (2) a low order of toxicity to eye tissue, such that the solution can be used to treat a contact lens such that the lens can subsequently be placed on the eye without rinsing the solution from the lens. While challenging to develop, it would be especially desirable to obtain a Chemical Disinfecting Solution that could be used for soft contact lenses and that would allow direct placement of a contact lens on an eye following soaking in the solution and/or rinsing and rewetting with the solution. Such a product may provide increased efficacy, resulting in greater protection to the lens wearer against infection caused by microorganisms, while providing maximum convenience. Finally, it would be desirable for the biocidal efficacy of the disinfecting solution to be sufficiently high to achieve efficacious disinfection, or at least not inherently inefficacious disinfection, of a contact lens with respect to bacteria and fungi in the event, for whatever reason, that the contact lens wearer does not carry out a regimen involving mechanical rubbing or the like using the contact-lens solution.